1. Field of the Invention
This invention is concerned with the manufacture of high quality lubricating oils, and in particular with lubricating oils derived from petroleum distillate fractions. It is especially directed to the preparation of low pour point lubricating oils that have a high V.I. (V.I. will be used herein to denote "viscosity index") from crude oils of high wax content. This invention is particularly directed to catalytically dewaxing a waxy distillate lubricating oil with a ZSM-5 zeolite catalyst characterized by large crystals to obtain a lubricating oil of low pour point and of high V.I.
2. Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types. This complexity and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
In general, the basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A refined lubricant stock may be used as such as a lubricant, or it may be blended with another refined lubricant stock having somewhat different properties. Or, the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and V.I. improvers. As used herein, the term "stock", regardless whether or not the term is further qualified, will refer only to a hydrocarbon oil without additives. The term "raw stock" will be used herein to refer to a viscous distillate fraction of crude petroleum oil isolated by vacuum distillation of a reduced crude from atmospheric distillation, and before further processing, or its equivalent. The term "solvent-refined stock" will refer to an oil that has been solvent refined, for example with furfural. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "waxy", as used herein will refer to an oil of sufficient wax content to result in a pour point greater than +20.degree. F. The term "stock", when unqualified, will be used herein generically to refer to the viscous fraction in any stage of refining, but in all cases free of additives.
Briefly, for the preparation of a high grade distillate lubricating oil stock, the current practice is to vaccum distil an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450 to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the dewaxed dissolved raffinate in quantity sufficient to provide the desired pour point for the subsequently recovered raffinate.
Other processes such as hydrofinishing or clay percolation may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
Viscosity index (V.I.) is a quality parameter of considerable importance for distillate lubricating oils to be used in automotive engines and aircraft engines which are subject to wide variations in temperature. This Index is a series of numbers ranging from 0 to 100 which indicate the rate of change of viscosity with temperature. A viscosity index of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100 and 210.degree. F., and referral to correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to it is meant the V.I. as noted in the Viscosity Index tabulations of the ASTM (D567), published by ASTM, 1916 Race Street, Philadelphia 3, Pa., or equivalent.
To prepare high V.I. automotive and aircraft oils the refiner usually selects a crude oil relatively rich in paraffinic hydrocarbons, since experience has shown that crudes poor in paraffins, such as those commonly termed "naphthene-base" crudes yield little or no refined stock having a V.I. above about 40. (See Nelson, supra, pages 80-81 for classifications of crude oils). Suitable stocks for high V.I. oils, however, also contain substantial quantities of waxes which result in solvent-refined lubricating oil stocks of high pour point, i.e., a pour point substantially greater than +20.degree. F. Thus, in general, the refining of crude oil to prepare acceptable high V.I. distillate stocks ordinarily includes dewaxing to reduce the pour point to not greater than +20.degree. F. The refiner, in this step, often produces saleable paraffin wax by-product, thus in part defraying the high cost of the dewaxing step.
Raw distillate lubricating oil stocks usually do not have a particularly high V.I. However, solvent-refining, as with furfural for example, in addition to removing unstable and sludge-forming components from the crude distillate, also removes components which adversely affect the V.I. Thus, a solvent refined stock prior to dewaxing usually has a V.I. well in excess of specifications. Dewaxing, on the other hand, removes paraffins which have a V.I. of about 200, and thus reduces the V.I. of the dewaxed stock.
In recent years catalytic techniques have become available for dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. No. Re. 28,398 (of U.S. Pat. No. 3,700,585) to Chen, et al is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such processes combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938. In U.S. Pat. No. 3,755,138 to Chen et al is described a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point. The entire contents of these patents are herein incorporated by reference.
It is interesting to note that catalytic dewaxing, unlike prior-art dewaxing processes, although subtractive, is not a physical process but rather depends on transforming the straight chain and other waxy paraffins to non-wax materials. The process, however, is more economical and thus of industrial interest, even though at least some loss of saleable wax is inherent. Commercial interest in catalytic dewaxing is evidence of the need for more efficient refinery processes to produce low pour point lubricants.
It is an object of this invention to provide a process of improved selectivity for catalytically dewaxing a hydrocarbon oil boiling within the range of 450.degree. to 1050.degree. F. It is another object of this invention to provide a method for removing paraffin waxes from a lubricating oil stock with reduced loss of V.I. These and other objects will become apparent to one skilled in the art on reading this entire specification including the claims appended hereto.